Field of the Invention
The invention relates to a signal analyzer. More specifically, the invention relates to a signal analyzer for nuclear magnetic flowmeters that is in a power signal path between a signal generator for generating an electric excitation signal and an electrical load. The signal analyzer includes a power signal line in the power signal path for the transmission of electrical signals. The signal analyzer also includes a decoupling circuit for decoupling a first signal characterizing the excitation signal occurring in the load and for decoupling a second signal characterizing part of the excitation signal reflected at the load. The signal analyzer further includes a first attenuator and a second attenuator and having a detector for determining the ratio of the magnitude of the first signal to the magnitude of the second signal and for determining the phase difference between the first signal and the second signal.
Description of Related Art
The atomic nuclei of the elements having nuclear spin also have a magnetic moment caused by the nuclear spin. Nuclear spin can be regarded as angular momentum and can be represented by a vector. Accordingly, the magnetic moment can also be represented by a vector, which is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus, in the presence of a macroscopic magnetic field, aligns itself parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of precession is the Larmor frequency ωL and is proportional to the magnitude of the magnetic field strength B. The Larmor frequency is calculated according to the gyromagnetic ratio, ωL=γ·B. γ, which is at a maximum for hydrogen nuclei. The precession of the magnetic moment of a nucleus is a magnetic alternating field at the Larmor frequency, which can induce an electrical alternating signal with the same frequency in a coil. Nuclear magnetic resonance measurement methods are measurement methods that influence the precession of atomic nuclei of a medium in the presence of a macroscopic magnetic field by a controlled magnetic field excitation means, and that evaluate the action of the influence.
Nuclear magnetic flowmeters are one example of measurement devices that use nuclear magnetic resonance. Nuclear magnetic flowmeters include nuclear magnetic measurement devices that can measure the flow rate (i.e., the flow velocity) of individual phases of a multiphase medium and the relative proportions of individual phases in the multiphase medium. Nuclear magnetic flowmeters can be used, for example, for measuring the flow rate of a multiphase medium that has been conveyed from oil sources. This medium consists essentially of the liquid phases of crude oil and salt water, and the gaseous phase natural gas. All such phases contain hydrogen nuclei, which are necessary for nuclear magnetic resonances and are excitable to different nuclear magnetic resonances.
A medium that has been conveyed from oil sources can also be measured using test separators. The conveyed medium is introduced into test separators over a time interval, wherein the test separator separates the individual phases of the medium from one another and determines the proportions of the individual phases in the medium. However, test separators, in contrast to nuclear magnetic flowmeters, are not able to reliably separate proportions of crude oil smaller than 5%. Since the proportion of crude oil of all oil sources continuously decreases, and since the proportion of crude oil of a host of oil sources is already less than 5%, at present, it is not possible to economically exploit these oil sources using test separators. In order to exploit oil sources with very small proportions of crude oil, correspondingly accurate flowmeters for mediums consisting of several phases are necessary. In particular, nuclear magnetic flowmeters are suitable for this purpose.
In nuclear magnetic flowmeters, the magnetic field, which initially aligns the magnetic moments of the nuclei of the medium and determines the Larmor frequency of precession, is generated by a magnetization device. The controlled magnetic field, which excites the aligned and precessed nuclei, can be generated by an electric coil. The coil is a component of the electrical load of the signal analyzer and the electric load is electrically connected to the signal generator of the nuclear magnetic flowmeter generating electrical excitation from the power signal line. Excitation signals oscillating at the Larmor frequency are particularly suitable for excitation. The electrical load is usually designed as a resonant circuit with an adjustable resonant frequency and an adjustable input impedance to allow for efficient transmission of power from the signal generator via the power signal line to the coil along the power signal path.
The resonant frequency of the electrical load can be attuned to the Larmor frequency of the phase of the medium to be measured for implementing the efficient transmission of power. Additionally, power adaptation can be performed in the power signal path. Power adaptation is provided by adjusting the output impedance of the signal generator, the characteristic impedance of the power signal line and the input impedance of the load to one another. A measure for power adaptation of the electrical load is the ratio between the power of the excitation signal occurring in the load and the part of the power of the occurring excitation signal reflected from the load. During power adaptation, no reflection of the excitation signal occurring in the load occurs on the load.
Signal analyzers known from the prior art, in particular for nuclear magnetic flowmeters, of the type described above have a decoupling circuit with a first directional coupler and a second directional coupler. The first directional coupler generates a first signal characterizing an excitation signal occurring in the load and the second directional coupler generates a second signal characterizing a part of the excitation signal reflected at the load. In this case, the phase position of the first signal relative to the phase position of the voltage of the excitation signal and the phase position of the second signal with respect to the phase position of the voltage of the reflected portion of the excitation signal are equal, wherein both voltages are measures for the power transmitted via the power signal path. If the output impedance of the signal generator has no reactance, which is usually the case, then in the signal analyzers known from the prior art and using conventional power signal lines, the phase difference between the first signal and the second signal needed for power adaptation is 0 degrees. Phase differences in the range of 0 degrees, however, influence evaluation by the detector.